1. Field of the Invention
The present invention relates to aluminum heat exchanges used in condensers and evaporators of automobile air conditioners, radiators, intercoolers, oil coolers, etc, in which a fin member and a working fluid passage member are bonded together by brazing, and further relates to aluminum alloy fin materials suitable for use in such aluminum heat exchangers. More specifically, the present invention relates to aluminum heat exchangers having excellent corrosion resistance and heat transfer characteristics and to aluminum alloy fin materials for such heat exchangers which have a superior combination of properties of high thermal conductivity and beneficial sacrificial anode effect after brazing.
2. Description of the Prior Art
In heat exchangers used in condensers and evaporators for automobile air conditioners, radiators, heaters, intercoolers, oil coolers and so forth, an aluminum alloy material constituting a working fluid passage member and an aluminum alloy material constituting a fin member are bonded together by brazing in which a brazing material is placed either on the fluid passage member or on the fin member. When the brazing material is placed on the fluid passage member, a bare fin material is used as the fin member and a composite material consisting of a core made of an aluminum alloy and a cladding layer made of an Al-Si alloy brazing filler material, which is applied onto one side or both sides of the core is used as the liquid passage material. When the brazing material is placed on the fin member, an extruded tube is used as the liquid passage material and a composite material consisting of a core made of an aluminum alloy and a cladding layer made of an Al-Si alloy brazing filler material, which is applied onto both sides of the core, is used as the fin member.
As the core material of the composite for the liquid passage material, Al-Mn alloys have been employed. As the material for the extruded tube, pure aluminum, for example, AA 1050, 1070 or 1100, or aluminum alloys containing up to about 0.5% Cu or Mn have been employed. The AA designation and number of aluminum alloys in the specification are those of the U.S. Aluminum Association.
The fin material is required to have a sacrificial anode effect to protect the foregoing liquid passage material or the foregoing extruded tube from corrosion as well as a good high-temperature buckling resistance sufficient to prevent the problems of deformation and erosion attack by the brazing filler material which may arise during brazing. Since the addition of Mn is effective for preventing the deformation and erosion attack by the brazing filler material, Al-Mn alloys, such as AA 3003 alloy or AA 3203 alloy, have been employed as the fin material or, when the fin material is constituted by a brazing sheet, as the core of such a brazing fin material.
Addition of Zn, Sn, In, etc., to the Al-Mn alloy fin material has been proposed to make the fin material electrochemically anodic to the fluid passage material and, thereby, provide a sacrificial anode property to the fin material. (Refer to Japanese Patent Publication No. 56-12395 and Japanese Patent Laid-Open No. 62-120455). Further, in order to obtain an improved buckling resistance (sagging resistance) at high temperatures, the addition of Cr, Ti, Zr and so on has been proposed in Japanese Patent Application Laid-Open Nos. 50-118919 and 54-61354). Heat exchangers fabricated from a combination of the foregoing extruded tube and fin members are, for example, described in Japanese Patent Publication No. 59-52760.
Further, applicants have previously proposed a heat exchanger fin material having a high strength and superior heat transfer properties which were achieved by an increased Fe addition without an accompanying addition of Mn. (Refer to Japanese Patent Application No. 1-218648.)
When the extruded tube material of the pure aluminum and the fin material of the Al-Mn alloy containing Zn, Sn, In, etc., are, as set forth above, combined, a certain extent of corrosion prevention can be expected in the tube material by the sacrificial anode effect of the fin material, but the corrosion prevention distance (i.e., the range in which the sacrificial anode effect of the fin material occurs) is short, since the potential difference between the tube material and fin material is insufficiently large. Consequently, there is the problem that the tube material is susceptible to pitting corrosion in the parts away from the fin material. Alternatively, when the extruded tube is made of the alloys containing Cu or Mn instead of the pure aluminum extruded tube, the electrochemical potential of the tube becomes noble and the potential difference between the tube and fin also becomes large. Consequently, the corrosion prevention range becomes wider. However, when the addition of Cu or Mn to the tube is increased, the extrudability of the tube, especially with regard to multivoid tubes, is deleteriously decreased. Therefore, the addition of Cu or Mn should be limited to up to 0.5% and the satisfactory solution to the above problem has not been achieved.
Recently, zinc-coated tubes have been increasingly employed as the material constituting the fluid passages in which the tubes are protected from corrosion by a Zn-diffusion zone formed during brazing. When such tubes are combined with the conventional fins made of, for example, the foregoing Al-Mn alloy with Zn, Sn, In, etc., by brazing, the formed Zn-diffusion zone corrodes in preference to the fins during service, because the Zn-diffusion zone has a greater anodic potential than the fins. Therefore, there is a problem of separation of the fins from the tubes.
Further, in recent years, there has been a strong demand for weight-reduction and cost-saving of heat exchangers. In order to respond to such a demand, it is necessary to reduce the thickness of the structural materials (e.g., working fluid passage material, fin material or the like) used in heat exchangers. However, when the thickness of the fin material is reduced, the cross section through which heat flux transfers is also reduced and serious problems will arise in the heat-transfer efficiency.
In order to eliminate these problems, it is desired that the fins after brazing have a high thermal conductivity. However., in the case of using the Al-Mn alloy as the fin material or as the core of the brazing fin material, Mn in solid solution increases due to the dissolution of Al-Mn compounds during the brazing operation, thereby resulting in a considerable reduction of the thermal conductivity. Further, in order to increase the thermal conductivity of the fin material, attempts have been made to use fin materials made of pure aluminum (such as AA 1050, AA 1070 or the like) with the addition of Zn, Sn, In, Cr, Ti, Zr or the like. However, such fin materials have a poor buckling resistance at high temperatures and their strength is reduced after brazing, although they have a high thermal-conductivity. Consequently, the fins constituted by such materials are apt to fall and the foregoing problems have not been substantially eliminated.